The present inventions pertain to the field of thermal control of circuits, including, more specifically, thermal control of circuit boards using resistive heaters.
Electronics deployed in outdoor environments are often subjected to wide fluctuations in ambient air temperature, barometric pressure, and relative humidity. These environmental fluctuations can cause the electronics, and particularly a circuit board and mounted electronics components, to attain a temperature below the dew point temperature of the surrounding ambient air, allowing moisture to condense on the cooler surfaces of the electronics. Additionally, as the container in which the electronics are enclosed xe2x80x9cbreathesxe2x80x9d (a result of changes in barometric pressure), moisture can be drawn into the container. The formation and collection of moisture on the electronics typically results in corrosion of the electronic components, thus having negative reliability implications.
Outdoor electronics are not only subjected to wide fluctuations in ambient air temperature, but are also subject to extremely cold ambient air temperature, adversely affecting transient startup of the electronics. As such, extreme operational temperature requirements are often imposed on outdoor electronics, making it necessary to purchase components that can function at temperatures as low as xe2x88x9240xc2x0 C. (xe2x88x9240xc2x0 F.). Industrial grade components are rated at this extreme temperature, but are much more expensive than commercial grade components. Oftentimes, industrial grade components are not even available.
One method of protecting outdoor electronics from moisture is to coat the electronics, i.e., the circuit board and associated electronic components, with a conformal coat material, which serves as a moisture barrier. Although the conformal coat reduces the rate at which moisture can come into contact with the electronics, the conformal coat is not completely impervious to moisture penetration. Additionally, this approach adds steps in the manufacturing process and has a negative impact on rework procedures. Also, if any moisture or contaminants are present on the electronics prior to application of the conformal coat, they can become trapped between the conformal coat and the electronics, in which case, corrosion of the electronics can begin immediately. Furthermore, the use of a conformal coat on the electronics does not address the requirement that the electronic components function at extreme temperature.
One method addressing this extreme temperature requirement is the use of external discrete heaters to heat the air surrounding the electronics. External discrete heaters, however, have not been previously used to prevent moisture from condensing on the electronics. Additionally, because the heaters are discrete, significant cost is added to the electronics fabrication process due to the need for external wiring, mounting hardware, additional assembly steps and stocking/storing of separate heaters. Also, because the heaters are exposed to the ambient temperature, their efficiency is reduced.
Another method addressing this extreme temperature requirement is the use of dedicated energized copper traces to provide heat to the circuit board prior to operation of the temperature sensitive electronics, as described in U.S. Pat. No. 5,896,259. The electrical resistivity of the copper traces, however, is minimal, and thus, to prevent the power source that supplies electrical energy to the copper traces from transmitting energy into a virtual short circuit, the length of the copper traces must be great enough to provide the required resistance thereto. Due to the length of these copper traces, a substantial area of the circuit board, and in some cases one or more layers of the circuit board, must be used to route the copper traces.
There thus is a need for an apparatus and method that provides a less costly and more efficient manner of thermally controlling electronic equipment during adverse environmental conditions.
The present inventions comprise a novel thermally controlled circuit board.
In a first aspect of the invention, a circuit board comprises a substrate, which can be either rigid or flexible. The circuit board further includes a heater. By way of nonlimiting example, the heater can include one or more planar resistive elements, which may be embedded in or disposed on the surface of the substrate. These planar resistive elements are composed of a substantially resistive material. That is, the planar resistive elements are not highly conductive. The circuit board further includes a power source, which is selectively coupled to the heater. The circuit board further includes thermal control circuitry, which is configured for selectively coupling the power source to the heater.
In accordance with another aspect of the present inventions, the circuit board includes a substrate, which can be either rigid or flexible. The circuit board further includes a heater comprising one or more resistive elements embedded within the substrate. These embedded resistive elements are composed of a substantially resistive material. That is, the planar resistive elements are not highly conductive. In the preferred embodiment, the one or more resistive elements are planar, but can be other than planar. In the preferred embodiment, the heater is configured to dissipate a level of heat that satisfies one or more predetermined thermal criteria. The predetermined thermal criteria can include, among other things, a set temperature of the substrate above an ambient temperature to which the substrate is exposed, or a time in which a measured temperature rises to a predetermined temperature level. The circuit board can include another heater comprising one or more resistive elements embedded within the substrate. The heaters can be advantageously used as low-power and high-power heaters, providing different levels of heat dissipation to the circuit board.
In accordance with still another aspect of the present inventions, the circuit board includes a substrate, which can be either rigid or flexible. The substrate can either be formed of a single substrate layer or multiple substrate layers. The circuit board further includes a heater comprising one or more planar resistive elements carried by the substrate. If the substrate is formed of multiple substrate layers, the planar resistive elements can be carried within the multiple substrate layers or carried external to the multiple substrate layers. By way of nonlimiting example, the planar resistive elements can be arranged in a bank, and the size and number of the resistive elements can be determined based on one or more predetermined thermal criteria.
In accordance with still another aspect of the present inventions, the circuit board includes a substrate, which can be either rigid or flexible. The circuit board further includes a heater carried by the substrate. The heater is configured for dissipating a level of heat required to prevent the condensation of moisture on the circuit board. The heater can comprise embedded resistive elements and/or planar resistive elements, or even external discrete heaters.